1. Field of the Invention
The present invention relates to a method for bending material which exhibits a large spring back at a normal temperature and is liable to harden during the machining and be easily crackable, such as a difficult-to-work metallic wire, particularly to a method for bending such a difficult-to-work metallic wire as titanium alloy including Ni--Ti type alloy having a shape-memory effect, hyper elastic characteristic or others. More specifically, the present invention relates to a method for shaping a coil section of a difficult-to-work metallic wire suitable for constituting an antenna for a mobile phone having a bend of small radius of curvature in a coil form.
2. Description of Related Art
Demand for small-size portable type telecommunication devices have recently increased and functions thereof have increasingly been improved. Together therewith, the minimization of an outer dimension of such a device has also been required, which is accompanied with the requirement for the improvement in function of antenna used therefor and for the minimization of its size. Also, since such an antenna is withdrawn from a body of the telecommunication device and maintained in an extended state upon use, it is necessary for the antenna not to easily bend during withdrawal and not to be broken even though it is brought into contact with an outer object in the extended state.
For satisfying the requirement for the minimization of a size of antenna while maintaining the resistance to flexing and the resistance to breakage at a desired level, it has been contemplated to constitute the antenna from a difficult-to-work metallic wire, for example, of titanium alloy including Ni--Ti alloy having a shape-memory effect, hyper elastic characteristic or others and to shape a coil section in this antenna.
In the prior art, the bending work of the difficult-to-work metallic wire is generally carried out by a so-called hot working wherein the wire is preliminarily heated as a whole to a predetermined temperature by the radiation heating using a heater, the high frequency induction heating or the resistance heating, and then is deformed in a die of a press.
This prior art bending method, however, has problems in that it is time-consuming and requires an odd space or a heat-resistant installation because the wire must be heated as a whole by either of the above-mentioned heating methods and/or a large amount of heat is irradiated to the environment surrounding the same as well as a precise and, therefore, expensive temperature sensor is necessary for the temperature control of the wire.
Also, among the above-mentioned heating methods, the resistance heating is capable of heating the wire alone in a shorter time with less influence on the environment because it is an internal heating utilizing the electric resistance of the wire itself. According to this heating method, however, it is difficult to evenly heat the wire in its entirety and to bend the wire into a complicated profile. Thus, this heating method is restricted to a bending process for obtaining a relatively simple profile.
In other words, there has heretofore been no technology suitable for bending the wire into a complicated profile such as a coil form as described above.